Today, piezoelectric substances such as Pb(Zr,Ti)O3 are used in mechanoelectric conversion elements for application in actuators and sensors. Such piezoelectric substances are formed into a thin film on a substrate of Si (silicon) or the like for expected application in MEMS (microelectromechanical system) elements.
MEMS elements can be manufactured by high-precision processing exploiting semiconductor processes such as photolithography, and thus can be made compact, at high density. In particular, forming a large number of MEMS elements simultaneously at high density on a comparatively large Si wafer with a diameter of 6 inches or 8 inches helps greatly reduce costs, compared with forming them individually.
Moreover, forming piezoelectric substances into thin films and forming devices into MEMSs help improve mechanoelectric conversion efficiency, and thereby brings additional benefits such as improved device sensitivity and other characteristics. For example, in heat sensors, forming them into MEMSs reduces their heat conductance, leading to higher measurement sensitivity; in ink-jet heads for printers, forming them into MEMSs increases nozzle density, allowing high-precision patterning.
One common material for a thin film of a piezoelectric substance (piezoelectric thin film) is the crystal composed of Pb, Zr, Ti, and O called PZT (lead zirconate titanate). PZT exhibits a good piezoelectric effect when it has a perovskite structure of the ABO3 type as shown in FIG. 18, and thus has to be made single-phase. The shape of the unit lattice of a PZT crystal having a perovskite structure changes with the ratio of Ti to Zr, which are the elements that can occupy site B. Specifically, a high proportion of Ti gives PZT a cubic crystal lattice, and a high proportion of Zr gives PZT a rhombohedral crystal lattice. With the mol ratio of Zr to Ti near 52:48, the two crystal structures coexist, and the phase boundary with such a composition ratio is called morphotropic phase boundary. An MPB composition provides optimal piezoelectric properties in terms of piezoelectric constant, value of polarization, dielectric constant, etc., and thus piezoelectric thin films of MPB compositions are preferentially used.
What is called piezoelectric effect here is the effect by which a piezoelectric substance deforms when a voltage is applied to it and by which a piezoelectric substance produces an electric field (potential difference) when deformed. FIG. 19 schematically shows how a piezoelectric substance exhibits the piezoelectric effect differently with different crystal orientations. When the piezoelectric substance is in (100) orientation, that is, when the direction P of polarization of the piezoelectric substance is taken as (100) direction and this direction is perpendicular to the substrate, applying an electric field in a direction perpendicular to the substrate permits, since the direction P of polarization of the piezoelectric substance is identical with the direction E of application of the electric field, the intensity of the electric field to be fully converted into the force deforming the piezoelectric substance, allowing efficient deformation of the piezoelectric substance in the direction perpendicular to the substrate. By contrast, when the piezoelectric substance is in (111) orientation, since the direction P of polarization of the piezoelectric substance, i.e., (100) direction, crosses the direction E of application of the electric field, the intensity of the electric field is not fully converted into the force deforming the piezoelectric substance, resulting in smaller deformation of the piezoelectric substance in the direction perpendicular to the substrate.
As discussed above, the piezoelectric properties of a piezoelectric substance change also with its crystal orientation, (100) orientation yielding better piezoelectric properties than (111) orientation. When a piezoelectric thin film is used as a MEMS actuator, it needs to be formed with a thickness of 3 μm to 5 μm to meet the required displacement producing force, and its driving requires good piezoelectric properties. This makes (100) orientation preferable as the crystal orientation of a piezoelectric thin film.
In some cases, however, (111) orientation or other crystal orientation can be preferable in terms of fatigue properties and ease of processing. Specifically, forming a piezoelectric thin film in (111) orientation is preferable to forming one in (100) orientation in that domain rotation is less likely during voltage application, making patterning by etching easier. In either case, controlling the crystal orientation of a piezoelectric thin film is essential to obtain stable properties.
A piezoelectric thin film can be formed on a substrate of Si or the like by chemical film formation, such as CVD (chemical vapor deposition), or by physical film formation, such as sputtering or ion plating, or by liquid-phase growth, such as sol-gel process.
An attempt to form a thin film of piezoelectric substance (with a thickness of several micrometers) on a Si substrate or the like often ends in a failure to obtain the desired properties. This can be attributed to residual stress due to a difference in lattice constant, or in coefficient of linear expansion, between the substrate or a lower electrode and the piezoelectric thin film, hampering formation of the perovskite structure and crystal orientation needed in the piezoelectric thin film.
As a remedy, according to a known technology, a primer layer (buffer layer, seed layer) is provided between a substrate and a piezoelectric layer to control the crystallinity of the piezoelectric layer. For example, according to Patent Document 1 identified below, a primer layer of PLT (lead lanthanate titanate) is provided between a substrate and a piezoelectric layer (e.g., PLZT, i.e., lead lanthanum zirconium titanate). PLT in the primer layer has a property of easily forming a perovskite crystal even on a Si substrate or a lower electrode. Thus, forming a piezoelectric layer on such a primer layer facilitates formation of the piezoelectric layer with a perovskite structure.
For another example, according to Patent Document 2 identified below, a piezoelectric layer (PZT) is composed of two layers, namely a first piezoelectric film and a second piezoelectric film, and a buffer layer of PLT is provided between a substrate and the piezoelectric layer. The PLT comprises columnar particles with a cross-sectional diameter of, e.g., 40 nm. In the piezoelectric layer, the cross-sectional diameter (e.g., 160 nm) of columnar particles forming the second piezoelectric film is larger than the cross-sectional diameter (e.g., 40 m) of columnar particles forming the first piezoelectric film, which is the closer to the buffer layer. With this structure, while adhesion of the piezoelectric layer is improved to prevent exfoliation, good piezoelectric properties are obtained.